TRK12/120 Restoration

[dtvmcdonald]

I got a picture on the thing tonight.
The sweeps seem to still be OK. The cathode and
grid signals to the CRT seem reasonable ...
say +75 on the cathode and -5 to +25 to +50 on the grid.

However the CRT G2 and focus seems totally out of whack.
With those K and G1 voltages I get normal brightness but very very
bad focus. If I either increase the video drive or decrease the
cathode voltage ... making it excessively bright ... I get good
focus but its very unstable.

I set the resistor chain on the HV to the ratios given by the
schematic ... the set came with the G2 set way up the
chain from what the schematic says.


Does anybody with one of these radar tubes know what the
correct G2 and focus voltages are?

Doug McDonald

[Steve McVoy]

The only difference between the radar tube and the 12AP4 voltages is the filament, which is 6.3 rather than 2.0. Focus and G2 are the same: about 250v for G2 and 2000v for focus.

[dtvmcdonald]

Let me get those voltages for sure right. They are referenced to the
cathode, yes???? So with the cathode at +75 the G2 should be at
75+250 = 325 and the focus should be adjustable around 2075, with
respect to ground.

The instability is perhaps due to bad resistors in the bleeder chain.
I will replace them today.

Close examination of the HV cable showed it to have cracked
rubber insulation. It was replaced.

I forgot to mention that the grid lead to the CRT was dead.
It used a very small wire, like #36 or so, presumably for
low capacitance. It was broken. I replaced it with #24 wire.

[Steve McVoy]

Those voltages are absolute - not relative to the cathode. They are from the factory manual schematic.

I'm sure you have checked the 7500v line?

Focus voltage won't be precisely 2000, and in some cases I've had to change the divider to get a voltage that will produce a focused picture.

[dtvmcdonald]

You are right about the 2000v focus ... its very wrong. The resistor
chain values on the schematic give more like 1400 and result in
correct focus near the center of the control.

I changed all the resistors to modern carbon ones, using two or three
2-watt ones in series for each of the originals to get enough voltage
rating.

I have no idea if this helped, however.


That's because I discovered that the focus wire to the
original socket was open. It was NOT OPEN yesterday morning, because I
measured it when making the adapter for the radar tube. But it
sure is now. How can a wire go open? This is not a small gauge wire,
its like #22 stranded. I did not cut open the bundle it is in but replaced
it with a new HV wire cable-tied to the bundle.

I did cut about 1/2 inch off it at the socket end and 2 inches inside
the chassis. I stripped the ends expecting to find that the shortened
wire was OK. No, its open. How can such a wire be open?

In any case the set now produces a quite nice picture. Its not without
defects. First is what looks like interference but probably
is beats between the audio and color carriers. I've got to figure
out where the audio trap is and check its adjustment.

Second is blurring due to too much capacitance from the grid lead.
This varies with placement. I will make new cables with 0.005 inch
wire, and also make spacers to keep it away from the rest
of the bundles.

Other than that, the set now works correctly.

[dtvmcdonald]

Part of the blurring and the interference lines were only on ch.4, not 3. The lines
indeed were audio-color beating.

The cause was a misadjusted RF coil in the tuner. Now both 3 and 4 produce nice pictures.

[Steve McVoy]

Great news. Congratulations on getting it working.

I'm sure you know that with the low IF frequencies this set has horrible image rejection. I never attempt to feed them with anything but a single channel (3 or 4).

[dtvmcdonald]

I have a rubber mask for this thing that came with the cabinet.

However, this morning I tried fitting it in the cabinet and on the tube
and it appears to have shrunk. It would seriously restrict the
view compared to the area of the wood mask. Is shrinkage normal
or at least precedented?

If it is not normal, and no proper replacements I exist I can of course
kludge something out of foam.

[Steve D.]

This ad on the ETF classifieds page:
Reproduction TRK-12 rubber CRT masks, $100 a piece. Contact Barry Kasindorf, barry at barrykasindorf dot com 508-528- 0026.

Don't know if it is still current

-Steve D.

[Steve McVoy]

Depends on what you mean by shrinking. Attached are a couple of photos that show what a good one looks like.

[tvdude1]

Originally Posted by Steve D.

This ad on the ETF classifieds page:
Reproduction TRK-12 rubber CRT masks, $100 a piece. Contact Barry Kasindorf, barry at barrykasindorf dot com 508-528- 0026.

Don't know if it is still current

-Steve D.

I know him well he still has some left.

[bkasindorf]

My ears were burning..Here are some pictures of the TRK-12 masks I have. I have 2 left.
-Barry

[dtvmcdonald]

I got all the parts in the cabinet safely and it works. The mask is indeed
the right size.

All that's left is a permanent bucking transformer, which I have plenty of.
I'm currently using a variac.

The only problem I had in putting it in the cabinet is that the
picture was rotated 180 degrees. I didn't feel like removing the tube to rotate the yoke
so I reversed the leads instead. The la
st 1/4 inch of the leads were somewhat dicey
anyway so this had an added bonus of safety.

Another TRK12 lives.

[baursam]

I'd LOVE to see some pics if at all possible!!

[dtvmcdonald]

There is not much more to show than what is already posted.
Look earlier in the thread.

You are not getting a photo of the current underside of the TV
power chassis! First, I forgot to take one before installing it
in the cabinet. Second, some people might puke if they saw it,
all those triples of modern Allen-Bradley style 2 watt carbon resistors
replacing the originals. I had to buy the type of resistors that
our EE department storeroom stocked since I wanted them
in a hurry. I could post a picture of the removed (and 30-50% high resistance) originals.

Its not ready for a frontal shot, running, as the knobs for
it were only put "in the mail" yesterday.
There is no back.

However, I should and will post a picture of the HV
connector I built for it. As I said, I have not run a lathe for
somewhere between 15 and 20 years [dedicated to the
memory of George Pisello who taught me], but it turned
out quite nice. There was lots of work with a boring bar.
I will also post a picture of the HV rectifier running in
the perforated sheet metal cage I made for it.